1. Field of the Invention
The invention relates to improved blister packages. More particularly, the invention pertains to multilayered films for forming improved blister packages having a lid-stock film which is capable of being heat sealed directly to fluoropolymer films, including polychlorotrifluoroethylene (PCTFE) homopolymers or copolymers.
2. Description of the Related Art
It is known to produce multilayered structures to take advantage of different properties exhibited by the various layers in the structures. Typical of multilayered structures are multilayered films in which different layers have specific characteristics. These films may then be used in packaging applications having customized properties that may be required for a particular packaged product. For example, films which are used to protect food, medicines, paints, adhesives, biomaterials, chemicals, etc., require properties such as good thermal and environmental stability, UV blocking characteristics, transparency in visible light wavelengths and an excellent moisture barrier.
In many packaging applications it is desirable to utilize fluoropolymer films which are particularly desirable because they have an excellent moisture barrier property. This property is extremely important when the films are used to form packages for products intended for human consumption. Most fluoropolymers, especially PCTFE and ethylene-chlorotrifluoroethylene (ECTFE), exhibit such excellent moisture barrier properties, making them exceptionally good polymers as barrier packaging material.
In forming packaging films, such fluoropolymers are typically laminated with other polymeric films to form multilayered structures, with only a thin layer of the fluoropolymer needed to take advantage of the desirable properties. However, one significant problem is that fluoropolymers do not adhere strongly to most other polymers. In fact, many fluoropolymers are known for their non-stick characteristics. Various attempts have been made to produce multilayer fluoropolymer films. For example, multilayered structures wherein at least one layer is a fluoropolymer and the other layer is a thermoplastic homopolymer or copolymer, have been used for the production of highly oriented, dimensionally stable fluoropolymer films. In such structures, a tie layer or intermediate adhesive layer is necessary to bond the film layers. Multilayer films containing PCTFE and thermoplastic polymer layers are also known. See, for example, U.S. Pat. Nos. 5,874,035 and 5,945,221. However, the structures of the multilayered films are limited because of the inability of fluoropolymer films to sufficiently stick to certain lid-stock film materials, even with an intermediate adhesive layer.
In a typical prior art multilayered blister packaging film, a fluoropolymer layer is attached to a polymeric base layer, e.g. polyvinyl chloride, via an intermediate adhesive layer. A lid-stock film comprised of a polyethylene terephthalate layer attached to an aluminum foil is then adhesively sealed to the base layer to encapsulate a product within the package. This structure has been found to provide sufficient adhesion between the polymeric base layer and the lid-stock film. This structure is also quite effective in preventing the transmission of water vapor and other moisture from the blister side of the film and from the lid-stock side of the package. However, one significant problem associated with prior art blister package multilayered film is that a considerable amount of moisture also enters into the package from between the layers by seeping in through the sides of the package. This is because while fluoropolymer films are known to exhibit excellent moisture barrier properties, other thermoplastic polymers such as polyesters or polyolefins typically have poor moisture barrier properties. Therefore, one potential solution would be to use multiple fluoropolymer films in a package. However, this is a very costly solution to the problem as fluoropolymers are quite expensive. Another potential solution would be to eliminate the non-fluoropolymer films from the structure altogether. However, while the other thermoplastic layers may not provide a good moisture barrier, they do provide other desirable characteristics, such as good mechanical strength, which are necessary for a final product. Furthermore, one additional problem remains the fact that the adhesives of the prior art have not been able to sufficiently adhere fluoropolymer films to other thermoplastic polymer layers in multilayered films. Accordingly, thus far, no prior art structure has functioned to prevent moisture from entering the package through the sides.
Preventing moisture from entering the package from the sides is a problem which is equally important as preventing moisture from entering from a top or bottom surface of the package. Unfortunately, this problem is not appreciated by users of prior art packaging structures. To be sure, the failure to sufficiently prevent all moisture from seeping into a package will cause the product stored within the package to be compromised. It is therefore desirable to provide a multilayered structure having an improved moisture barrier between a PCTFE or other fluoropolymer layer and another polymeric layer, which is capable of blocking substantially all moisture from entering a blister package. The present invention provides an effective solution to this compelling need.